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T291004 Lääketieteelliset mittauslaitteet:
Ultraääni lääketieteessä
Lähteinä on käytetty:
• Wikipedia
• Sprawls Educational Foundation: The Physical Principles of Medical Imaging
(www.sprawls.org)
1. Medical ultrasonography
Diagnostic sonography (ultrasonography) is an ultrasound-based diagnostic imaging
technique used to visualize subcutaneous body structures including tendons, muscles,
joints, vessels and internal organs for possible pathology or lesions. Obstetric
sonography is commonly used during pregnancy and is widely recognized by the public.
There is a plethora of diagnostic and therapeutic applications practiced in medicine.
In physics the term "ultrasound" applies to all acoustic energy with a frequency above
human hearing (20,000 hertz or 20 kilohertz).
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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1.1. Diagnostic applications
Orthogonal planes of a 3 dimensional sonographic volume with transverse and coronal
measurements for estimating fetal cranial volume. [1], [2]
Typical diagnostic sonographic scanners operate in the
frequency range of 2 to 18 megahertz, hundreds of
times greater than the limit of human hearing. The
choice of frequency is a trade-off between spatial
resolution of the image and imaging depth: lower
frequencies produce less resolution but image deeper
into the body.
Sonography (ultrasonography) is widely used in
medicine. It is possible to perform both diagnosis and
therapeutic procedures, using ultrasound to guide interventional procedures (for instance
biopsies or drainage of fluid collections). Sonographers are medical professionals who
perform scans for diagnostic purposes. Sonographers typically use a hand-held probe
(called a transducer) that is placed directly on and moved over the patient.
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Ultraääni lääketieteessäSonography is effective for imaging soft tissues of the body. Superficial structures such
as muscles, tendons, testes, breast and the neonatal brain are imaged at a higher
frequency (7-18 MHz), which provides better axial and lateral resolution. Deeper
structures such as liver and kidney are imaged at a lower frequency 1-6 MHz with lower
axial and lateral resolution but greater penetration.
Medical sonography is used in the study of many different systems:
System Description See also
Cardiology
Echocardiography is an essential tool in
cardiology, to diagnose e.g. dilatation of parts
of the heart and function of heart ventricles and
valves
see echocardiography
Endocrinology
Gastroenterology
In abdominal sonography, the solid organs of
the abdomen such as the pancreas, aorta,
inferior vena cava, liver, gall bladder, bile ducts,
kidneys, and spleen are imaged. Sound waves
are blocked by gas in the bowel, therefore there
are limited diagnostic capabilities in this area.
The appendix can sometimes be seen when
inflamed eg: appendicitis.
Gynaecologysee gynecologic
ultrasonography
Neurology
for assessing blood flow and stenoses in the
carotid arteries (Carotid ultrasonography) and
the big intracerebral arteries
see Carotid
ultrasonography.
Intracerebral: see
Transcranial Doppler
Obstetrics
Obstetrical ultrasound is commonly used during
pregnancy to check on the development of the
fetus.
see obstetric
ultrasonography
Ophthalmology
see A-scan
ultrasonography, B-
scan ultrasonographyUrology to determine, for example, the amount of fluid
retained in a patient's bladder. In a pelvic
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OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Ultraääni lääketieteessäsonogram, organs of the pelvic region are
imaged. This includes the uterus and ovaries or
urinary bladder. Men are sometimes given a
pelvic sonogram to check on the health of their
bladder and prostate. There are two methods of
performing a pelvic sonography - externally or
internally. The internal pelvic sonogram is
performed either transvaginally (in a woman) or
transrectally (in a man). Sonographic imaging
of the pelvic floor can produce important
diagnostic information regarding the precise
relationship of abnormal structures with other
pelvic organs and it represents a useful hint to
treat patients with symptoms related to pelvic
prolapse, double incontinence and obstructed
defecation.[3]
Musculoskeletal tendons, muscles, nerves, and bone surfaces
Cardiovascular
system
To assess patency and possible obstruction of
arteries Arterial sonography, diagnose DVT
(Thrombosonography) and determine extent
and severity of venous insufficiency
(venosonography)
Intravascular
ultrasound
Other types of uses include:
• Emergency Medicine ; many applications, including the Focused Assessment with
Sonography for Trauma (FAST) exam for assessing significant hemoperitoneum
or pericardial tamponade after trauma.
• Intervenional ; biopsy, emptying fluids, intrauterine transfusion [disambiguation needed]
(Hemolytic disease of the newborn)
• Contrast-enhanced ultrasound
A general-purpose sonographic machine may be able to be used for most imaging
purposes. Usually specialty applications may be served only by use of a specialty
transducer. Most ultrasound procedures are done using a transducer on the surface of
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Ultraääni lääketieteessäthe body, but improved diagnostic confidence is often possible if a transducer can be
placed inside the body. For this purpose, specialty transducers, including endovaginal,
endorectal, and transesophageal transducers are commonly employed. At the extreme
of this, very small transducers can be mounted on small diameter catheters and placed
into blood vessels to image the walls and disease of those vessels.
1.2. Therapeutic applications
Therapeutic applications use ultrasound to bring heat or agitation into the body.
Therefore much higher energies are used than in diagnostic ultrasound. In many cases
the range of frequencies used are also very different.
• Ultrasound may be used to clean teeth in dental hygiene.
• Ultrasound sources may be used to generate regional heating and mechanical
changes in biological tissue, e.g. in occupational therapy, physical therapy and
cancer treatment. However the use of ultrasound in the treatment of
musculoskeletal conditions has fallen out of favor.[4][5]
• Focused ultrasound may be used to generate highly localized heating to treat
cysts and tumors (benign or malignant), This is known as Focused Ultrasound
Surgery (FUS) or High Intensity Focused Ultrasound (HIFU). These procedures
generally use lower frequencies than medical diagnostic ultrasound (from 250
kHz to 2000 kHz), but significantly higher energies. HIFU treatment is often
guided by MRI.
• Focused ultrasound may be used to break up kidney stones by lithotripsy.
• Ultrasound may be used for cataract treatment by phacoemulsification.
• Additional physiological effects of low-intensity ultrasound have recently been
discovered, e.g. its ability to stimulate bone-growth and its potential to disrupt the
blood-brain barrier for drug delivery.
• Procoagulant at 5-12MHz
1.3. From sound to image
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Ultraääni lääketieteessäThe creation of an image from sound
is done in three steps - producing a
sound wave, receiving echoes, and
interpreting those echoes.
Sound wave is typically produced by
a piezoelectric transducer encased
in a probe [disambiguation needed]. Strong,
short electrical pulses from the
ultrasound machine make the
transducer ring at the desired
frequency. The frequencies can be anywhere between 2 and 18 MHz. The sound is
focused either by the shape of the transducer, a lens in front of the transducer, or a
complex set of control pulses from the ultrasound scanner machine (Beamforming). This
focusing produces an arc-shaped sound wave from the face of the transducer. The wave
travels into the body and comes into focus at a desired depth.
Older technology transducers focus their beam with physical lenses. Newer technology
transducers use phased array techniques to enable the sonographic machine to change
the direction and depth of focus. Almost all piezoelectric transducers are made of
ceramic.
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Materials on the face of the transducer
enable the sound to be transmitted
efficiently into the body (usually seeming
to be a rubbery coating, a form of
impedance matching). In addition, a
water-based gel is placed between the
patient's skin and the probe.
The sound wave is partially reflected from
the layers between different tissues. Specifically, sound is reflected anywhere there are
density changes in the body: e.g. blood cells in blood plasma, small structures in organs,
etc. Some of the reflections return to the transducer.
1.3.1. Receiving the echoes
The return of the sound wave to the transducer results in the same process that it took to
send the sound wave, except in reverse. The return sound wave vibrates the transducer,
the transducer turns the vibrations into electrical pulses that travel to the ultrasonic
scanner where they are processed and transformed into a digital image.
1.3.2. Forming the image
The sonographic scanner must determine three things from each received echo:
1. How long it took the echo to be received from when the sound was transmitted.
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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T291004 Lääketieteelliset mittauslaitteet:
Ultraääni lääketieteessä2. From this the focal length for the phased array is deduced, enabling a sharp
image of that echo at that depth (this is not possible while producing a sound
wave).
3. How strong the echo was. It could be noted that sound wave is not a click, but a
pulse with a specific carrier frequency. Moving objects change this frequency on
reflection, so that it is only a matter of electronics to have simultaneous Doppler
sonography.
Once the ultrasonic scanner
determines these three things, it can
locate which pixel in the image to light
up and to what intensity and at what
hue if frequency is processed (see
redshift for a natural mapping to hue).
Transforming the received signal into
a digital image may be explained by
using a blank spreadsheet as an
analogy. We imagine our transducer is a long, flat transducer at the top of the sheet. We
will send pulses down the 'columns' of our spreadsheet (A, B, C, etc.). We listen at each
column for any return echoes. When we hear an echo, we note how long it took for the
echo to return. The longer the wait, the deeper the row (1,2,3, etc.). The strength of the
echo determines the brightness setting for that cell (white for a strong echo, black for a
weak echo, and varying shades of grey for everything in between.) When all the echoes
are recorded on the sheet, we have a greyscale image.
For computational details see also: Confocal laser scanning microscopy, Radar, Echo
sounding
1.4. Sound in
the body
Ultrasonography (sonography) uses
a probe containing one or more
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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T291004 Lääketieteelliset mittauslaitteet:
Ultraääni lääketieteessäacoustic transducers to send pulses of sound into a material. Whenever a sound wave
encounters a material with a different density (acoustical impedance), part of the sound
wave is reflected back to the probe and is detected as an echo. The time it takes for the
echo to travel back to the probe is measured and used to calculate the depth of the
tissue interface causing the echo. The greater the difference between acoustic
impedances, the larger the echo is. If the pulse hits gases or solids, the density
difference is so great that most of the acoustic energy is reflected and it becomes
impossible to see deeper.
The frequencies used for medical imaging are generally in the range of 1 to 18 MHz.
Higher frequencies have a correspondingly smaller wavelength, and can be used to
make sonograms with smaller details. However, the attenuation of the sound wave is
increased at higher frequencies, so in order to have better penetration of deeper tissues,
a lower frequency (3-5 MHz) is used.
Seeing deep into the body with sonography is very difficult. Some acoustic energy is lost
every time an echo is formed, but most of it (approximately ) is
lost from acoustic absorption.
The speed of sound is different in different materials, and is dependent on the acoustical
impedance of the material. However, the sonographic instrument assumes that the
acoustic velocity is constant at 1540 m/s. An effect of this assumption is that in a real
body with non-uniform tissues, the beam becomes somewhat de-focused and image
resolution is reduced.
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OAMK Tekniikan yksikkö Hyvinvointiteknologia
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To generate a 2D-image, the ultrasonic beam is swept. A transducer may be swept
mechanically by rotating or swinging. Or a 1D phased array transducer may be use to
sweep the beam electronically. The received data is processed and used to construct
the image. The image is then a 2D representation of the slice into the body.
3D images can be generated by acquiring a series of adjacent 2D images. Commonly a
specialised probe that mechanically scans a conventional 2D-image transducer is used.
However, since the mechanical scanning is slow, it is difficult to make 3D images of
moving tissues. Recently, 2D phased array transducers that can sweep the beam in 3D
have been developed. These can image faster and can even be used to make live 3D
images of a beating heart.
Doppler ultrasonography is used to study blood flow and muscle motion. The different
detected speeds are represented in color for ease of interpretation, for example leaky
heart valves: the leak shows up as a flash of unique color. Colors may alternatively be
used to represent the amplitudes of the received echoes.
1.5. Modes of sonography
Four different modes of ultrasound are used in medical imaging[6]. These are:
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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T291004 Lääketieteelliset mittauslaitteet:
Ultraääni lääketieteessä• A-mode: A-mode is the
simplest type of ultrasound.
A single transducer scans a
line through the body with the
echoes plotted on screen as
a function of depth.
Therapeutic ultrasound
aimed at a specific tumor or
calculus is also A-mode, to
allow for pinpoint accurate
focus of the destructive wave energy.
• B-mode: In B-mode ultrasound, a linear array of transducers simultaneously
scans a plane through the body that can be viewed as a two-dimensional image
on screen.
• M-mode: M stands for motion. In m-mode a rapid sequence of B-mode scans
whose images follow each other in sequence on screen enables doctors to see
and measure range of motion, as the organ boundaries that produce reflections
move relative to the probe.
• Doppler mode: This mode makes use of the Doppler effect in measuring and
visualizing blood flow
• 3, 4 and 5-mode
1.6. Doppler sonography
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OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Sonography can be enhanced with Doppler measurements, which employ the Doppler
effect to assess whether structures (usually blood) are moving towards or away from the
probe, and its relative velocity. By calculating the frequency shift of a particular sample
volume, for example a jet of blood flow over a heart valve, its speed and direction can be
determined and visualised. This is particularly useful in cardiovascular studies
(sonography of the vascular system and heart) and essential in many areas such as
determining reverse blood flow in the liver vasculature in portal hypertension. The
Doppler information is displayed graphically using spectral Doppler, or as an image
using color Doppler (directional Doppler) or power Doppler (non directional Doppler).
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Ultraääni lääketieteessäThis Doppler shift falls in the audible range and is often presented audibly using stereo
speakers: this produces a very distinctive, although synthetic, pulsing sound.
Most modern sonographic machines use pulsed Doppler to measure velocity. Pulsed
wave machines transmit and receive series of pulses. The frequency shift of each pulse
is ignored, however the relative phase changes of the pulses are used to obtain the
frequency shift (since frequency is the rate of change of phase). The major advantages
of pulsed Doppler over continuous wave is that distance information is obtained (the time
between the transmitted and received pulses can be converted into a distance with
knowledge of the speed of sound) and gain correction is applied. The disadvantage of
pulsed Doppler is that the measurements can suffer from aliasing. The terminology
"Doppler ultrasound" or "Doppler sonography", has been accepted to apply to both
pulsed and continuos Doppler systems despite the different mechanisms by which the
velocity is measured.
It should be noted here that there are no standards for the display of color Doppler.
Some laboratories insist on showing arteries as red and veins as blue, as medical
illustrators usually show them, even though, as a result, a torturous vessel may have
portions with flow toward and away relative to the transducer. This can result in the
illogical appearance of blood flow that appears to be in both directions in the same
vessel. Other laboratories use red to indicate flow toward the transducer and blue away
from the transducer which is the reverse of 150 years of astronomical literature on the
Doppler effect. Still other laboratories prefer to display the sonographic Doppler color
map more in accord with the prior published physics with the red shift representing
longer waves of echoes (scattered) from blood flowing away from the transducer; and
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Ultraääni lääketieteessäwith blue representing the shorter waves of echoes reflecting from blood flowing toward
the transducer. Because of this confusion and lack of standards in the various
laboratories, the sonographer must understand the underlying acoustic physics of color
Doppler and the physiology of normal and abnormal blood flow in the human body.See: [7][8] [9] [10]
1.7. Contrast media
The use of microbubble contrast media in medical sonography to improve ultrasound
signal backscatter is known as contrast-enhanced ultrasound. This technique is currently
used in echocardiography, and may have future applications in molecular imaging and
drug delivery.
1.8. Attributes
As with all imaging modalities, ultrasonography has in list of positive and negative
attributes.
1.8.1. Strengths
• It images muscle, soft tissue, and bone surfaces very well and is particularly
useful for delineating the interfaces between solid and fluid-filled spaces.
• It renders "live" images, where the operator can dynamically select the most
useful section for diagnosing and documenting changes, often enabling rapid
diagnoses. Live images also allow for ultrasound-guided biopsies or injections,
which can be cumbersome with other imaging modalities.
• It shows the structure of organs.
• It has no known long-term side effects and rarely causes any discomfort to the
patient.
• Equipment is widely available and comparatively flexible.
• Small, easily carried scanners are available; examinations can be performed at
the bedside.
• Relatively inexpensive compared to other modes of investigation, such as
computed X-ray tomography, DEXA or magnetic resonance imaging.
Copyright © Jukka Jauhiainen 2009
OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Ultraääni lääketieteessä• Spatial resolution is better in high frequency ultrasound transducers than it is in
most other imaging modalities.
1.8.2. Weaknesses
• Sonographic devices have trouble penetrating bone. For example, sonography of
the adult brain is very limited though improvements are being made in
transcranial ultrasonography.
• Sonography performs very poorly when there is a gas between the transducer
and the organ of interest, due to the extreme differences in acoustic impedance.
For example, overlying gas in the gastrointestinal tract often makes ultrasound
scanning of the pancreas difficult, and lung imaging is not possible (apart from
demarcating pleural effusions).
• Even in the absence of bone or air, the depth penetration of ultrasound may be
limited depending on the frequency of imaging. Consequently, there might be
difficulties imaging structures deep in the body, especially in obese patients.
• The method is operator-dependent. A high level of skill and experience is needed
to acquire good-quality images and make accurate diagnoses.
• There is no scout image as there is with CT and MRI. Once an image has been
acquired there is no exact way to tell which part of the body was imaged.
1.9. Risks and side-effects
Ultrasonography is generally considered a "safe" imaging modality.[11] However slight
detrimental effects have been occasionally observed (see below). Diagnostic ultrasound
studies of the fetus are generally considered to be safe during pregnancy. This
diagnostic procedure should be performed only when there is a valid medical indication,
and the lowest possible ultrasonic exposure setting should be used to gain the
necessary diagnostic information under the "as low as reasonably achievable" or ALARA
principle.
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OAMK Tekniikan yksikkö Hyvinvointiteknologia
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Ultraääni lääketieteessäWorld Health Organizations technical report series 875(1998).[12] supports that
ultrasound is harmless: "Diagnostic ultrasound is recognized as a safe, effective, and
highly flexible imaging modality capable of providing clinically relevant information about
most parts of the body in a rapid and cost-effective fashion". Although there is no
evidence ultrasound could be harmful for the fetus, US Food and Drug Administration
views promotion, selling, or leasing of ultrasound equipment for making "keepsake fetal
videos" to be an unapproved use of a medical device.
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OAMK Tekniikan yksikkö Hyvinvointiteknologia
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